Most explosives are characterized by high nitrogen and oxygen content and low carbon and hydrogen content. They are also usually of high density. Aware of this, the FAA began funding tests of thermal neutron activation (TNA) in 1985. TNA involves the use of a radioactive source such as Californium-252 which emits neutrons. The neutrons are slowed or moderated in materials high in hydrogen such as polyethylene (at which stage the neutrons are "thermalized") and are then absorbed by the object of interest. The absorption leads to the emission of gamma rays which are characteristic of the elements present.
Analysis of these rays provides information as to the nitrogen content of the object bombarded. While explosives characteristically have a high nitrogen content, so do other materials such as certain plastics, silk and nylon which are commonly contained in luggage. Unfortunately TNA screening devices cannot distinguish between explosive materials and these non-explosive materials and so false alarms are often raised which can cause considerable delays. Also, TNA scanning devices require a very intense neutron source and extreme measures are needed to shield airport staff and travellers from the radiation.
The sensitivity of these devices is also less than desirable but improving that would increase the incidence of false alarms. Furthermore, TNA scanning devices are about the size of a small car, can weigh of the order of 10 tonnes and cost about $1 million each. Also, it has been suggested that between 300 and 700 such units would be required to deal with the demands of large international airports in the United States at a cost of $500 million for the machines plus housing and operating costs of at least $92 million a year.
Dual beam X-ray machines are being field tested. They can detect organic materials, such as explosives, with one beam and inorganic materials, such as metals, with the other beam.
Hand-held vapour sniffers are also being tested. These take in air and identify molecules in terms of their vapour pressure, atomic weight and liquid solubility by chromographic means.
Plastic explosives such as Semtex however have a lower vapour pressure than TNT so they can be difficult to detect by such means.
Computerised tomography or CT scanning commonly used in medical diagnosis and research has been applied to the problem by scanning for an object's density, total mass and indicating its atomic number and composition. But as Dr. Grodzin of MIT explained at an international meeting (International Conference on Accelerators in Industry and Research, Denton, Tex., Nov. 5-9, 1990): "More than a dozen nuclear-based techniques have been proposed for rapidly scanning airport luggage to find hidden explosives by measuring their elemental distributions. In most almost every scheme, the technological challenge is the accelerator, which must produce its intense beams of neutrons and photons . . . in an airport environment, perhaps even in an airport concourse".